R-1: Townsley et al., U.K. Patent 1,193,349. PA0 R-2: W. J. Priest, Research Disclosure, December, 1977, "Process for Preparing Stable Aqueous Dispersion of Certain Hydrophobic Materials," pp. 75-80. PA0 R-3: T. J. Chen et al. (Kodak), U.S. Pat. Nos. 4,199,363; 4,214,047; 4,133,687; 4,127,499; 4,203,716; 4,247,627; and 4,127,499. PA0 R-4: L. K. J. Tong (Kodak), U.S. Pat. Nos. 2,852,386 and 2,772,163. PA0 R-5: O. Takahashi, (Fuji), European Patent Application 0,256,531. PA0 R-6: R. Matcjeck (Gaevert), German Patent 3,520,845. PA0 R-7: T. C. Webb et al. (Ciba-Geigy), U.S. Pat. No. 4,490,461. PA0 R-8: K. Nakazyo et al., (Fuji), U.S. Pat. No. 4,120,725. PA0 R-9: M. Yoneyama et al., (Fuji), U.S. Pat. No. 4,198,478. PA0 R-10: J. Minamizono et al., (Fuji), U.S. Pat. No. 4,291,113. PA0 R-11: Y. Mukunoki et al., (Fuji), U.S. Pat. No. 4,569,905. PA0 R-12: R. G. Mowrey et al., "Color Dispersions in Synthetic Polymer Vehicles," Research Disclosure, 15131, p. 42-43 (1976). PA0 R-13: K. Tokitou et al., (Konishiroku), U.S. Pat. No. 4,358,533. PA0 R-14: N. Fujiwhara et al., (Konishiroku), U.S. Pat. No. 4,368,258. PA0 R-15: L. K. J. Tong, (Kodak), Canada 542,135. PA0 R-16: Mitsubishi Paper Mill, Great Britain 1,456,278. PA0 R-17: P. Bagchi et al., "Preparation of Low Viscosity Small-Portable Photographic Dispersions in Gelatin," U.S. patent application Ser. No. 366,397. PA0 R-18: B. Chu, "Laser Light-Scattering," Academic Press, 1974, New York. PA0 R-19: Anonymous, "Photographic Silver Halide Emulsions, Preparations, Addenda, Processing and Systems," Research Disclosure, 308, p. 933-1015 (1989). PA0 R-20: T. H. James, "Theory of Photographic Process," 4th Ed., McMillan (1977). PA0 R-21: T. Brandrup et al., "Polymer Handbook," John Wiley, New York (1974). R-22: P. Bagchi et al., "Methods of Forming Stable Dispersions of Photographic Materials," U.S. patent application Ser. No. 297,005.
It has been known in the photographic arts to precipitate photographic materials, such as couplers, from solvent solution. The precipitation of such materials can generally be accomplished by a shift in the content of a water miscible solvent and/or a shift in pH. The precipitation by a shift in the content of water miscible solvent is normally accomplished by the addition of an excess of water to a solvent solution. The excess of water, in which the photographic component is insoluble, will cause precipitation of the photographic component as small particles. In precipitation by pH shift, a photographic component is dissolved in a solvent that is either acidic or basic. The pH is then shifted such that acidic solutions are made basic or basic solutions are made acidic in order to precipitate particles of the photographic component which is insoluble at that pH. United Kingdom Patent 1,193,349-Townsley et al. (R-1) discloses a process wherein an organic solvent, aqueous alkali solution of a color coupler is mixed with an aqueous acid medium to precipitate the color coupler. It is set forth that the materials can either be utilized immediately, or gelatin can be added to the dispersion and chilled and remelted for use at a later date. In an article in Research Disclosure, December, 1977, entitled "Process for Preparing Stable Aqueous Dispersions of Certain Hydrophobic Materials", pages 75-80, by William J. Priest (R-2), it is disclosed that color couplers can be formed by precipitation of small particles from solutions of the couplers in organic auxiliary solvents. However, many coupler dispersions prepared in this manner are photographically very inactive compared to conventional dispersions prepared by milling procedures that contain coupler solvents.
It has been shown that when coupler molecules are imbibed into latex particles by dissolving the coupler in a water-miscible solvent, adding this to the latex and removing the solvent, the resultant dispersion produces adequate photographic activity (R-3 and R-4) for photographic utility. It seems that the polymer latex acts as a coupler solvent; however, such loading procedure requires very large quantities of solvent, which makes this procedure very expensive and hazardous for industrial production. In general such procedure is limited to a load of 3 part coupler and 1 part latex polymer. Prior art (R-5) indicates that polymerization or incorporation of a polymer into mechanically ground dispersions with no permanent solvent produces coupler dispersions that give very stable dye images. Also, incorporation of polymer into the photographic layer produces images of high dye stability as indicated in (R-6). Therefore, it is not clear as to whether the polymer needs to remain in the coupler particle or just in the photographic layer to produce the observed dye stability.
In (R-7), Webb et al. describes a process of dispersion preparation by homogenization of a solid solution of a photographic component and a polymer into aqueous gelatin solution by milling procedures. In the process of this invention, a photographic agent and a polymer is dissolved in a solvent. The solvent is then evaporated off to obtain a solid solution. The solid solution is then dispersed in aqueous gelatin by conventional milling procedures. In a specific embodiment this photographic compound is cross-linked to this polymer. This, in some cases is done by a cross-linking agent. The cross-linking may be done via a carboxyl group pendent on the polymer molecule. It is also known that conventional dispersion of photographic couplers can be prepared with some photographic advantages that contain both coupler solvent and a synthetic polyacrylamide polymer (R-8). In an alternate embodiment of this invention some water soluble acrylamide polymers can be added in aqueous phase along with gelatin for achieving added stability. Surfactant like polymers containing --SO.sub.3 H groups in phenol formaldehyde resins (R-9, R-11) and in acrylate polymers (R-10) have been used to stabilize milled conventional dispersions. Other polymeric vehicles have also been incorporated in photographic layers as gelatin replacement material (R-12).
Other solvent loading techniques like Chen's (R-3) have been described Tokitou et al. (R-13) and (R-14). (R-13) describes a process and composition where a photographic material is loaded into a polymer particle by using a large volume of water miscible solvent comprising a polymerized oligomeric material. In a special embodiment, the oligomeric material is polymerized in the presence of the photographic component to form a latex loaded composition. The process of latex loading in (R-14) is quite similar to Chen et al. (R-3). Tong (R-15) describes a very inefficient method of loading of couplers into latex dispersion by stirring the coupler for long periods of time with the latex and filtering off the excess coupler. This procedure led to less than 1 g of coupler per 20 g of the latex polymer in many cases. (R-16) describes loading of ultraviolet radiation absorbing compounds into polymer resin by the use of both permanent and auxiliary solvents in the presence of gelatin.
There are drastic differences between this invention and that of Chen (R-3). In this invention, the coupler is solubilized and the latex is swollen by base and a water miscible solvent, in contrast with Chen's (R-3) process where coupler solubilization and latex swelling are done by a water miscible solvent alone. In the present invention, the impregnation of this latex by the coupler is achieved by the neutralization by acid, whereas in the case of Chen, it is achieved by evaporative removal of the solvent. As Chen's method is a solvent shift method, it requires a large amount of water miscible (auxiliary) solvent. By Chen's (R-3) process the amount of solvent needed is between 15 to 20 times the weight of the coupler to be imbibed. This is a major drawback of Chen's procedure. In Chen's process the maximum loading is 3 parts coupler to 1 part polymer, whereas higher loading would be desirable. Chen's method requires at least 2% by weight of the monomers to be of the type that forms a water soluble polymer. A process that does not have any such requirement would be desirable.